1. Field of the Invention
The present invention relates to an operational amplifying circuit having an operational amplifier, and a push-pull circuit.
2. Description of the Related Art
Conventional operational amplifying circuits each having a push-pull circuit disposed at its output stage are widely known, one of which is shown in FIG. 3 for example.
In the above configuration, the operational amplifier 111 changes each base voltage of each of the NPN transistor 121 and PNP transistor 122 according to the input signals from its input terminals 111a and 111b, driving a load (not shown) connected to the output terminal 102a of the output circuit 102.
When the output voltage of the operational amplifier 111 has the low level, the transistor 121 turns off and the transistor 122 reversely turns on so that the transistor 122 pulls the current from the load connected to the output terminal 102a. 
In contrast, when the output voltage of the operational amplifier 111 has the high level, the transistor 121 turns on and the transistor 122 reversely turns off so that the transistor 121 pushes the current into the load connected to the output terminal 102a. 
The configuration of the operational amplifying circuit, however, while the output voltage of the operational amplifier 111 turns from the low level to the high level, or from the high level to the low level, the transistors 121 and 122 simultaneously turn off, respectively, when the output voltage of the operational amplifier 111 turns the voltage level close to the intermediate voltage (VCC/2) between the supply voltage VCC of the power source PS and the ground voltage, causing the output waveform of the current outputted through the output terminal 102a to be discontinuous.
An occurrence of the state that the transistors 121 and 122 simultaneously completely turn off so that no currents are outputted through the output terminal 102a of the output circuit 102 until either the transistor 121 or the transistor 122 turns on again causes this discontinuity also called “switching distortion”.